In fiber-optic communication system development, increasing the capacity of a fiber transport connection has been a continuing goal. In early optical communications systems virtually all transport was implemented by On-Off Keying (OOK) of a single wavelength, and throughput was upgraded by increasing the symbol rate. This was followed by the introduction of parallelization into the fiber link by carrying many different data streams on separate wavelengths in the same optical fiber using wavelength-division multiplexing (WDM). Currently, wavelength counts in practical deployments have coalesced around 80-96 wavelengths per fiber at 50 GHz spacing, and have stopped increasing rapidly. Other sophisticated modulation formats being used in evolving fiber optic communication systems include Polarization-Multiplexed Quadrature Phase-shift Keying (PM-QPSK) and Orthogonal Frequency-Division Multiplexing (OFDM), which are used to simultaneously achieve data rates higher than the symbol rates and improved spectral efficiency.
A typical optical fiber is made up of concentric cylinders of glass and other materials. FIG. 1 illustrates a conventional optical fiber 100. As illustrated in FIG. 1, at the center of the optical fiber 100 is the core 102, which is a region of high refractive index where the electromagnetic field of the light is concentrated. Surrounding the core 102 is the cladding 104, typically a region of lower refractive index than the core. The diameters and refractive indices of the core 102 and the cladding 104 are chosen so that the light is trapped by the core 102 and will not leak out of the fiber 100 as it propagates lengthwise along the fiber 100. The outermost layer, the coating 106, is applied to provide mechanical and chemical protection of the cladding 104 from scratches, micro-bends, water penetration, etc. The core 102, cladding 104, and coating 106 layer may be all be contained in an outer jacket (not shown), which provides additional mechanical strength and protection to the fiber 100. A typical single-mode fiber may have a 9 micron diameter core made of germanium (Ge)-doped silica glass, a 125 micron diameter cladding made of undoped silica glass, and a 250 micron diameter coating of sophisticated polymer compounds. Because of the small core diameter, only a single transverse mode of the light is possible.
Multimode fibers have a much larger core diameter, typically 50 or 63.5 microns, so they can support many transverse modes in the same core. Each transverse mode has a characteristic pattern of light distribution across the core and light is easily coupled from one mode to another, leading to a complex situation when light must be launched from a laser or extracted for a receiver. Although one can conceive of SDM transmission across a conventional multimode fiber, this approach can lead to strong and unstable mode coupling, with energy transferred from mode to mode as the fiber bends or shifts with time. In this case, the transceivers at the endpoints of such links would have to be quite complex and expensive, because the transceivers would be required to sort out the fluctuating energy transfer, which is a very computation-intensive process. For certain coupling coefficients, the ultimate channel capacity may be severely compromised by the mode coupling. In addition, all-optical add/drop of individual modes from such strongly-coupled channels is problematic, requiring optical mode coupling compensators which are not currently available. Further, it is likely that different spatial modes in a multimode fiber will need to have their powers rebalanced along a long transmission path, and this rebalancing will be very difficult when the modes are strongly mixed.
Accordingly, there is a need for multicore fibers in which mode coupling is kept very small over many hundreds of kilometers of transmission distance. A multicore fiber is an optical fiber having multiple separate cores embedded in a cladding region, and each core provides a separate spatial mode for propagating optical signals. Experiments with fibers that have seven separate cores within a cladding region of fairly conventional size (130 um diameter) show that modes can remain fairly well-separated over ˜10 km distance, at a wavelength of 1310 nm. However, at 1490 nm, core-to-core crosstalk introduces significant system penalties, and the data suggests that in the C-band near 1550 nm, such penalties would be severe. This core-to-core crosstalk will become still more severe as the number of cores in a multicore fiber increases and the separation between the cores decreases.